FIG. 1 illustrates a differential amplifier with conventional gain booster circuit. The first gain boosted cascode of the circuit is implemented by introducing a feedback transistor M3 to a cascode consisting of a common source transistor M1 associated with a negative input 102 and a common gate transistor M2. The second gain boosted cascode of the circuit is implemented by introducing a feedback transistor M3′ to a cascode consisting of a common source transistor M1′ associated with a positive input 104 and a common gate transistor M2′.
As illustrated in FIG. 1, the sources of the common source transistors M1 and M1′ are coupled to a tail current source realized by a bias voltage source Vbias0 biasing a tail transistor M0 to generate a tail current 110. A positive output node 106, which is coupled to the drain of the common gate transistor M2, is coupled to a current source I1 and a negative output node 108, which is coupled to the drain of the common gate transistor M2′, is coupled to a current source I1′. Since the number of cascodes is limited by the supply voltage Vdd, the feedback transistors M3 and M3′ are introduced to the cascode circuit to generate an additional gain. The gain boosted cascodes illustrated in FIG. 1 boosts the gain of the cascode circuit by increasing its output impedance (e.g., by the gain of the feedback transistor M3 or M3′).
In FIG. 1, the sources of the feedback transistors M3 and M3′ are coupled to the ground, and their drains are coupled to current sources I2 and I2′, respectively. This layout becomes problematic as the size of transistors is getting smaller. If a typical transistor with the minimum channel length of 0.12 um is implemented as the feedback transistor, the gate to source voltage becomes very small since the threshold voltage is very small in such device. In that case, the voltage at the drain of the source transistor M1 or M1′ (e.g., 0.3 volt) is not big enough to keep the tail current 110 in saturation mode as the tail transistor M0 and/or the common source transistor M1 is placed in the triode region. This may result in the substantial decrease in the gain which can be realized by the gain booster circuit if the components were maintained in the saturation mode.
FIG. 2 illustrates a differential amplifier with another example of conventional gain booster circuit. In this example, instead of connecting the source of the feedback transistors M3 to the ground, the source is connected to a tail current via a tail transistor M4 after the feedback transistor M3 is paired with a matching transistor M5. The gate of the matching transistor M5 is coupled to a voltage source Vbias5, and the drain of the matching transistor M5 is connected to a current source I3. The tail current via the tail transistor M4 is biased by a voltage source Vbias4. Thus, a desired voltage at the drain of the common source transistor M1 can be obtained by setting the voltage sources Vbias4 and/or Vbias5. The same arrangement is made to the right side of the symmetrical circuit as illustrated in FIG. 2.
To operate the gain boosted cascodes illustrated in FIG. 2, the sum of the drain to source voltage of the tail transistor M4 and the gate to source voltage of the feedback transistor M3 and the drain to source voltage of the common gate transistor M2 needs to be less than the voltage at a positive output voltage node 206. It is common for a typical positive supply voltage Vdd being 1.65 volts to 1.8 volts to split among stacked devices which are represented by the current source I1. The stacked devices may be 3 PMOS transistors stacked or the second stage of the differential amplifier. In such a case, the positive output voltage may be set at 0.75 volts.
However, the voltage at the source terminal of the feedback transistor M3 needs to be 200 mV or higher to maintain the tail transistor M4 in saturation mode. Then, it would be hard to keep the voltage at the source terminal of the common gate transistor M2, which is the sum of the drain to source voltage of the tail transistor M4 and the gate to source voltage of the feedback transistor M3, low enough to maintain the drain to source voltage of the common gate transistor M2 in saturation mode. This would result in the loss of gain. The same goes for the right side of the symmetrical circuit as illustrated in FIG. 2.
To sum it up, if the source terminal of the feedback amplifier M3 or M3′ is grounded, the small drain to source voltage of the common source transistor M1 or M1′ decreases the gain of the gain boosted cascode circuit. Conversely, if the source terminal of the feedback amplifier M3 or M3′ is connected to its own tail current source, the small drain to source voltage of the common gate transistor M2 or M2′ decreases the gain of the gain boosted cascode circuit.